The clustering coefficient of a selected node is defined as the probability that two randomly selected neighbors are connected to each other.
With the number of neighbors as n and the number of mutual connections between the neighbors r the calculation is:

The number of possible connections between two neighbors is n!/(2!(n-2)!) = 4!/(2!(4-2)!) = 24/4 = 6,
where n is the number of neighbors n = 4 and the actual number r of connections is 1.
Therefore the clustering coefficient of node 1 is 1/6.